Layar combines GPS, camera, and compass to identify your surroundings and overlay information on screen, in real time. It is available for Android now and it will be available for iPhone soon, but exclusively for the 3GS.

The reason is that Layar needs a compass to work, as Maarten Lens-FitzGerald—from developer SPRXmobile—tells us:

We are definitely going for the new iPhone 3GS because of the compass! We're aiming for release after summer, but we depend on Apple accepting it…

Yes, all those science fiction stories are now passe. I’m looking forward to when they incorporate the facial recognition module …